This invention relates to brushless DC motors that are electronically commutated. More particularly, it relates to a wave-wound, ironless, brushless DC fan motor that is of simple construction, inexpensive to manufacture, and reliable.
A typical goal for manufacturing a fan motor is to make one that is very simple and, consequently, has a low production cost. Side armature AC motors come close to achieving these goals. Recently, however, DC fan motors have become increasingly attractive, particularly for fans used to cool electronic circuits where DC power is available.
Brushless DC motors using Hall effect devices to sense the commutation points as the rotor rotates are well known in the art. One or more stator coils are repeatedly energized or have their energization reversed to effect a relocation of the magnetic field produced by the stator coil or coils. A permanent magnet rotor is continually attracted by the new magnetic field. For commutation, one or more Hall effect devices sense the location of the poles of the permanent magnet rotor to control the energization of the stator coil or coils, or a Hall effect device detects the position of one or more commutation magnets mounted to rotate with the rotor and provided especially to indicate, by changing the state of the Hall effect device, the commutation points as the rotor turns.
Many brushless DC motors have been complex in both their structure and their commutation circuitry, with a concomitant production cost. In situations where simple, low-cost, reliable fan motors have been needed, these brushless DC motors--which might, more appropriately, have been used for precision disc or tape drives, for instance--have been too expensive for the simple purpose of fan rotation.
Commonly owned, copending U.S. Pat. No. 4,563,622 of C. Deavers and J. Reffelt, incorporated by reference herein, discloses a simple brushless DC fan motor. The motor has an annular permanent-magnet rotor that is radially magnetized (polarized) in circumferential segments. Alternate circumferential segments have opposite polarities. The motor also has a stator with an electromagnetic structure including a coil, wound on a bobbin, a core, and a pair of arms terminating in pole pieces. The electromagnetic structure is located in a compartment at one location at the bottom of a generally circular housing. A coil is energized to produce a magnetomotive force that exerts a torque for turning the rotor. A commutation circuit, which includes a position detector, preferably a Hall effect device, selectively energizes the coil. The position detector detects the position of the rotor with respect to the stator and supplies a signal for controlling the commutation circuit.
Commonly owned U.S. Pat. No. 4,553,075 of Fred Brown and Alan Grouse discloses a further brushless DC motor with an annular permanent magnet rotor magnetized in oppositely polarized segments to present alternate opposite poles to an external electromagnet structure located at one location along the circumference of the rotor. In this motor two coils are wound in bifilar fashion and are alternately oppositely energized to reverse the field presented by pole pieces in magnetic conducting relation to the coils.
The DC fan motors disclosed in U.S. Pat. Nos. 4,563,622 and 4,553,075 are simply constructed, inexpensively manufactured, and reliable. The electromagnetic structure and coil are arranged so that there is a resultant radial magnetic force on the rotor by virtue of the electromagnetic structure being located at one location along the periphery of the permanent magnet. For example, in the arrangement illustrated in FIGS. 1 and 2 of U.S. Pat. No. 4,563,622, when the stator coil is energized, the magnetic field created produces a radial forces directed towards the stator magnetic pole pieces at one side of the rotor. When the stator coil is deenergized, these radial forces are removed. The resultant of these radial forces is a radial force in one radial direction each time the stator coil is energized. For very quiet operation, free of vibration, and having less demanding bearing requirements, it would be desirable to provide the counterbalancing or more nearly counterbalancing radial forces that multiple coils located about the entire periphery of a rotor can provide while still providing many of substantial benefits of the simple, inexpensive and reliable motor of the aforesaid application. In that case, in the design and manufacture of the motor, attention to vibration due to repeated radial forces on the rotor could be substantially reduced.